Method for extracting gallium from fly ash

ABSTRACT

Disclosed is a method for extracting gallium from fly ash, which comprises the following steps: crushing the fly ash and removing Fe by magnetic separation; then dissolving it by using hydrochloride acid ( 2 ) to obtain hydrochloric acid leachate; adsorbing gallium in the hydrochloric acid leachate with macro-porous cationic resin, followed by eluting to obtain the eluent ( 5 ) containing gallium; adding sodium hydroxide ( 6 ) solution into the eluent containing gallium to react and obtaining sodium metaaluminate solution containing gallium ( 8 ); introducing CO 2  into the sodium metaaluminate solution containing gallium ( 8 ) for carbonation, followed by separating gallium from aluminum and obtaining aluminum-gallium double salt ( 15 ) with the gallium to alumina mass ratio being more than 1:340; adding the aluminum-gallium double salt ( 15 ) into sodium hydroxide ( 17 ) to react, followed by alkalinity-adjustment concentration to obtain alkali solution containing gallium and aluminum; electrolyzing ( 10 ) the alkali solution containing gallium and aluminum to obtain metal gallium ( 11 ). The method simplifies the process and improves extraction efficiency of gallium.

TECHNICAL FIELD

The present invention relates to a method for extracting metal galliumfrom fly ash and in particular relates to a method for extracting metalgallium from circulating fluidized-bed fly ash.

BACKGROUND

Gallium is an important semiconductor material and widely used. Theprice of gallium is very high in the international market and thusgallium has a bright prospect. However, the reserve of gallium is low,only approximately 0.015% in the earth's crust. Gallium almost does notform minerals, but exists with other minerals in form of isomorphism.Therefore, extraction of gallium is considerably difficult. Gallium isoften found in conjunction with aluminum and zinc in minerals in nature.As such, sulfide deposits of zinc and bauxite ore serve as a primarysource of the extraction of gallium. Nowadays, more than 90% of galliumin the world is extracted from the by-product of alumina industry inwhich bauxite is used as a main raw material. The mother liquid used forthe enrichment and separation of gallium is the mother liquid obtainedfrom carbon precipitation (or seed precipitation) during the process forproducing alumina. The main component of such mother liquid obtainedfrom carbon precipitation (or seed precipitation) is a base sodiummetaaluminate solution containing gallium. Main methods for extractinggallium from such base solution include a method for removing aluminumvia lime cream and carbonation, dealumination method of carbonated limemilk two-stage decomposition method, precipitation method and resinadsorption method which develops in recent years.

The recent studies have shown that the fly ash obtained from some placescontains a large amount of gallium which even overpasses the galliumlevel of mineral deposit. It has been verified by researches that thegallium content in the fly ash is usually 12-230 μg/g. As compared withthe gallium contents of other resources, the fly ash deserves to beextracted for metal gallium as a raw material. In light of differentconditions of calcinations, the fly ash is classified into pulverizedcoal-fired boiler fly ash and circulating fluidized-bed fly ash. Thepulverized coal-fired boiler fly ash is produced when coal is burned ata very high temperature (1400-1600° C.), in which alumina is in glassystate or present as a mineral form of mullite crystals or corundumcrystals of hot aluminum mineral which make such alumina very stable.While the combustion temperature of circulating fluidized-bed fly ash ismuch lower than that of traditional pulverized coal-fired boiler flyash, only about 850° C. Different combustion temperatures make asubstantial difference in phase composition between the pulverizedcoal-fired boiler fly ash and circulating fluidized-bed fly ash, thatis, amorphous kaolinite enters into the main phase composition of thecirculating fluidized-bed fly ash, in which silicon dioxide, alumina andferric oxide or the like possess excellent activity.

CN 200810051209.5 discloses a method for extracting both alumina andgallium from fly ash. In the method, sodium metaaluminate solutioncontaining gallium is obtained by acid-leaching and alkali-leachingprocesses, and then gallium is enriched and separated via multiple-stagecarbon precipitation-sodium hydroxide dissolution process.

CN 200710065366.7 discloses a method for extracting silicon dioxide,alumina and gallium oxide from high-alumina fly ash. The methodcomprises steps of treating the residues produced after the extractionof silicon dioxide from fly ash to obtain sodium metaaluminate solutioncontaining gallium, using such solution as the mother liquid to enrichgallium via multiple-stage carbon precipitation-sodium hydroxidedissolution process and resin adsorption process.

CN 200710145132.3 discloses a method for co-producing gallium andalumina. The method comprises steps of treating fly ash to obtain sodiummetaaluminate solution containing gallium, enriching gallium by theBayer dissolving system and then separating the enriched gallium byadsorption process using chelating resin.

CN 200710141488.X discloses a method for producing gallium. Theintermediate product, i.e. mother liquid of carbon precipitation,obtained from the process for producing alumina from fly ash is used asa raw material and reacts with sodium bicarbonate, and then subjects toa thorough carbonation, so as to obtain a gallium concentrate.

In the above patent documents, the mother liquid of carbon precipitation(or seed precipitation) obtained from the process for producing aluminafrom fly ash is used as a raw material for the enrichment and separationof gallium, that is, the mother liquid used for extracting gallium is abase sodium metaaluminate solution containing gallium.

CN 200810017872.3 discloses a process for extracting gallium from flyash and coal gangue. In the process, an adsorption method via absorbentcolumns is used for extracting gallium from an aluminum chloridesolution containing gallium which is obtained by mixing fly ash andsodium carbonate, subjecting the mixture to calcination followed bywater leaching and carbon precipitating and then reacting withhydrochloride acid. Such process, as fly ash and sodium carbonate aremixed and calcined at a very high temperature before acid leaching, issuitable for extracting gallium from pulverized coal-fired boiler flyash which has weak activity.

Jiazhen He et al. has reported “a research on technique of recyclinggallium from fly ash” (Scientific Research, 2002, No. 5, p23-26), inwhich the fly ash reacts directly with hydrochloride acid to yield analuminum chloride solution containing gallium, without being calcined ata very high temperature, and then gallium is extracted by resinadsorption. The reaction temperature of the fly ash and hydrochlorideacid is low (60° C.), which makes the leaching efficiency of galliumvery low (35.2%). Moreover, the resin for extraction used in the methodis levextred resin (CL-TBP). The adsorptin principle of such resin issimilar to that of solvent extraction. Such resin is obtained bypolymerizing and curing the active group of an extracting agent with thebase resin. Consequently, the adsorption efficiency of the resin is verylow and the production cost is very high.

SUMMARY OF THE INVENTION

The object of the invention is to provide an improved method forextracting metal gallium from circulating fluidized-bed fly ash.

The method for extracting metal gallium from circulating fluidized-bedfly ash according to the invention comprises the following steps:

a) crushing the fly ash to a size of 100 mesh or smaller, removing ironby wet magnetic separation, such that the ferric oxides content in thefly ash is reduced to 1.0 wt % or less, then adding hydrochloride acidinto the de-ironed fly ash for acid-leaching reaction, and subjectingthe reaction product to solid-liquid separation, so as to yield ahydrochloric leachate having a pH value in the range of 1-3;

b) adsorbing gallium in the hydrochloric leachate by passing the samethrough a column loading with a macro-porous cationic resin; eluting thecolumn with water or hydrochloride acid as an eluting agent when theadsorption reaches saturation to obtain a gallium-containing eluent;

c) adding sodium hydroxide solution into the gallium-containing eluent,separating precipitates after reaction by filtration to remove iron inthe eluent and thus obtaining a gallium-containing sodium metaaluminatesolution;

d) subjecting the gallium-containing sodium metaaluminate solution tocarbonation by introducing carbon dioxide therein, followed byseparating gallium from most aluminum and obtaining gallium-aluminumdouble salt with the mass ratio of gallium to alumina being more than1:340; and

e) adding the obtained gallium-aluminum double salt into a sodiumhydroxide solution, followed by subjecting the reactant to evaporationand concentration to obtain a base solution containing gallium andaluminum with the contents of gallium and alumina being 1 mol/l or morerespectively, and then electrolyzing the base solution to obtain metalgallium.

Hereinafter the method according to the invention will be furtherdescribed in detail, but the present invention is not limited thereto.

In step a) according to an embodiment of the invention, the fly ashincludes, but is not limited to circulating fluidized-bed fly ash. Inlight of particle size distribution of the fly ash, the fly ash iscrushed to a size of 100 mesh or smaller, removing iron contained in thecrushed fly ash before the acid-leaching, such that the iron content inthe fly ash is reduced to 1.0 wt % or less. The methods for removingiron may be any conventional methods for removing iron, such as magneticseparation. Preferably, wet magnetic separation is used in the presentinvention. Any conventional magnetic separator suitable for removingiron from powder-like material may be used for the wet magneticseparation in the present invention, as long as the iron content of thefly ash can be reduced to 1.0 wt % or less. The iron content in the flyash is calculated on the basis of the weight of the dried fly ashcontaining no water.

Preferably, the magnetic separator used for fly ash is a vertical ringmagnetic separator. Further preferably, the vertical ring magneticseparator comprises a rotating ring, an inductive medium, an upper ironyoke, a lower iron yoke, a magnetic exciting coil, a feeding opening, atailing bucket and a water washing device, in which the feeding openingis used for feeding the coal ash to be de-ironed, the tailing bucket isused for discharging the non-magnetic particles after de-ironing, theupper iron yoke and the lower iron yoke are respectively arranged at theinner and outer sides of the lower portion of the rotating ring, thewater washing device is arranged above the rotating ring, the inductivemedium is arranged in the rotating ring, the magnetic exciting coil isarranged at the periphery of the upper iron yoke and the lower iron yokeso as to make the upper iron yoke and the lower iron yoke to be a pairof magnetic poles for generating a magnetic field in the verticaldirection, and the inductive medium is layers of steel plate meshes,each steel plate mesh is woven by wires, and the edges of the wires haveprismatic sharp angles.

Preferably, the upper iron yoke and the lower iron yoke are formedintegrally, and are arranged, in a plane perpendicular to the rotatingring, to surround the inner and outer sides of the lower portion of therotating ring.

Preferably, the vertical ring magnetic separator further comprises apressure balance chamber water jacket disposed adjacent to the magneticexciting coil.

Preferably, the steel plate mesh is made of 1Cr17.

Preferably, the magnetic exciting coil is a flat wire solenoid coilwhich is double glass envelope enamelled aluminum.

Preferably, the steel plate mesh has a medium layer spacing of 2-5 mm.More preferably, the steel plate mesh has a medium layer spacing of 3mm.

Preferably, the steel plate mesh has a thickness of 0.8-1.5 mm, a meshgrid size of 3 mmx 8 mm-8 mmx 15 mm, and a wire width of 1-2 mm. Morepreferably, the steel plate mesh has a thickness of 1 mm, a mesh gridsize of 5 mm×10 mm, and a wire width of 1.6 mm.

Preferably, the vertical ring magnetic separator further comprises apulsating mechanism, which is coupled with the tailing bucket via arubber plate.

Preferably, the inductive medium is provided in the entire circle of therotating ring.

When the above-said vertical ring magnetic separator is used formagnetic separation for de-ironing, it is necessary to timely test theiron content in the slurry subject to the magnetic separation. When theiron content in the slurry is equal to or lower than a predeterminedvalue, the slurry is discharged; when the iron content is higher thanthe predetermined value, the slurry is returned to the feeding openingfor further magnetic separation. Such magnetic separation may berepeated 2-4 times, preferably 2-3 times.

Preferably, when the slurry is magnetically separated by the verticalring magnetic separator, the vertical ring magnetic separator provides amagnetic field strength of 15,000 Gs or more, further preferably15,000-20,000 Gs, more preferably 15,000-17,500 Gs.

In step a) according to an embodiment of the invention, the filteredcake of the circulating fluidized-bed fly ash subject to magneticseparation is placed into an acid-resistant reactor and then thehydrochloride acid with a preferred concentration of 20-37 wt % is addedtherein to perform acid dissolving reaction. In a preferred embodiment,the molar ratio of HCl contained in the hydrochloride acid to aluminacontained in the fly ash is 4:1-9:1; the fly ash and hydrochloride acidreacts at a temperature in the range of 100-200° C. and under a pressurein the range of 0.1-2.5 MPa and the reaction time is 0.5-4.0 hours; andthen the reaction product is subjected to a solid-liquid separation andrinse to yield an hydrochloric leachate having a pH value in the rangeof 1-3. The process for the solid-liquid separation may be any ofconventional methods, such as settling separation, vacuum filtration,pressure filtration or centrifugation or the like.

In step b) according to an embodiment of the invention, saidmacro-porous cationic resin is preferably any one selected from D001,732, 742, 7020H, 7120H, JK008 and SPC-1.

In step b) according to an embodiment of the invention, saidmacro-porous cationic resin may be strong-acid-cationic resin, such asstyrene resins or acrylic resins. The essential performances of theresin include moisture content of 50.0-70.0%, exchange capacity of 3.60mmol/g or more, volume exchange capacity of 1.20 mmol/g or more, bulkdensity in wet state of 0.60-0.80 g/ml, particle size of 0.315-1.250 mm,available particle size of 0.400-0.700 mm and maximum workingtemperature of 95° C.

The gallium contained in the hydrochloric leachate obtained from step a)is adsorbed by passing the same through a column loading with themacro-porous cationic resin. The process for the adsorption may be anyof conventional methods. However, it is preferred to conduct theadsorption in such a way that the hydrochloric leachate passes throughthe resin column from bottom to top at 20-90° C., such that the acidleachate flows upwards piston-like in the gaps of the resin, with avolume flux of 1-4 times over resin volume per hour. The resin columnmay be single column or two cascaded columns. In the step, while galliumin the hydrochloric leachate is enriched by being absorbed by themacro-porous cationic resin, iron in the hydrochloric leachate issimultaneously effectively adsorbed, so that a refined aluminum chloridesolution with a low iron content is obtained, which can be then used forpreparing aluminum chloride crystal and metallurgical-grade alumina withlow iron content.

The macro-porous cationic resin may be eluted by an eluting agent toobtain a gallium-containing eluent when the adsorption reachessaturation. Preferably, the eluting agent is water or 2-10 wt %hydrochloride acid. The conditions of elution may include that theeluting temperature is 20-60° C., the amount of the eluting agent is 1-3times over the volume of the resin, the volume flux of the eluting agentis 1-3 times over resin volume per hour, and the eluting agent passesthrough the resin column in a top-in and bottom-out way during theelution.

The macro-porous cationic resin may regain adsorption capacity viaregeneration. The resin may be regenerated with 2-10 wt % hydrochlorideacid. During the regeneration, the temperature is 20-60° C., the amountof the hydrochloride acid is 1-2 times over the volume of the resin, andthe volume flux of the hydrochloride acid is 1-3 times over resin volumeper hour, the hydrochloride acid passes through said resin column in atop-in and bottom-out way.

In step c) according to an embodiment of the invention, sodium hydroxidesolution is added into the eluent under stirring and the mass ratio ofalumina in the eluent to sodium hydroxide is 1:1-2:1, the eluent reactswith the sodium hydroxide solution at 20-100° C., such that aluminumchloride and gallium chloride contained in the eluent react with sodiumhydrochloride to produce sodium metaaluminate/sodium metagallate andferric chloride precipitates in form of ferric hydroxide. The reactionproduct is subjected to a solid-liquid separation and rinse to yieldgallium-containing sodium metaaluminate solution. Preferably, theconcentration of sodium hydroxide solution used in step c) is 180-240g/l.

In step d) according to an embodiment of the invention, an appropriateamount of carbon dioxide may be fed into the gallium-containing sodiummetaaluminate solution, so as to conduct the carbonation once or severaltimes, till the mass ratio between gallium and alumina is more than1:340 in the obtained gallium-aluminum double salt. Particularly, thecarbonation(s) may comprise the following steps.

Primary carbonation: carbon dioxide is introduced with a flow rate of80-160 ml/min into the gallium-containing sodium metaaluminate mothersolution for a smooth carbonation, in which the reaction temperature iscontrolled to 40-90° C., the carbonation time is 4-10 h, the pH value atthe reaction end is 10.6-9.7. After the reaction, most aluminum isprecipitated in form of aluminum hydroxide, whereas gallium is retainedin the solution. The precipitate is removed from the solution, so as toseparate gallium and most aluminum for the first time;

Secondary carbonation: to the solution obtained from the primarycarbonation separating the aluminum hydroxide precipitates, carbondioxide is further introduced with a flow rate of 100-160 ml/min foradditional carbonation reaction, in which the reaction temperature iscontrolled to 30-60° C., the carbonation time is 3-7 h, the pH value atthe reaction end is 9.8-9.0, so as to precipitate all aluminum and mostgallium. The precipitate is separated by filtration to obtain agallium-aluminum double salt. The filtrate is concentrated byvaporization, and then sodium carbonate is crystallized out of thesolution. After removing the crystallized sodium carbonate, the solutioncontaining a small amount of gallium is recycled to the solutionobtained from the primary carbonation at the beginning of the secondarycarbonation.

If the mass ratio of gallium and alumina in the gallium-aluminum doublesalt obtained through the primary carbonation and the secondarycarbonation is equal to or less than 1:340, such double salt can bedissolved in sodium hydroxide solution or sodium metaaluminate mothersolution to conduct the primary carbonation and the secondarycarbonation again till the mass ratio of gallium and alumina in thegallium-aluminum double salt is more than 1:340. The gallium content ismeasures in accordance with the method of Standard of the People'sRepublic of China GB/T 20127.5-2006 “Steel and Alloy-Determination ofTrace Elements Contents Part V: Determination of Gallium Content byExtraction Separation-Rhodamine B Photometric Method”. The aluminumhydroxide content is calculated by 100% minus the measured galliumhydroxide content, which is then calculated to the alumina content. Inthe present invention, the aluminum hydroxide and sodium carbonateobtained from the steps for enriching and separating gallium can berecycled as by-product.

In step e) according to an embodiment of the invention, thegallium-aluminum double salt obtained from the secondary carbonation isadded into a sodium hydroxide solution to prepare the base solutioncontaining gallium and aluminum. Preferably, the concentration of thesodium hydroxide solution is 180-245 g/l. Both gallium content andsodium hydroxide content in the base solution are adjusted to 1 mol/l ormore by adjusting the alkalinity and/or concentrating. Then, the basesolution is electrolyzed with platinum electrodes used as the negativeand positive electrodes, electrolysis current of 180-200 mA/l,electrolysis voltage of 4V and electrolytic bath temperature of 35-45°C., so as to obtain the metal gallium product.

Preferably, the reaction temperature of the gallium-aluminum double saltprecipitate and the sodium hydroxide solution is 20-100° C.

In the present invention, the sodium salts contained in the electrolyzedsolution with a high content can be recycled by evaporation and theevaporated water can be re-used.

As compared with processes in the prior art, the method according to thepresent invention is simple, the extraction efficiency of gallium ishigh, the production coast is low, and the product quality is steady.The circulating fluidized-bed fly ash with high activity is adopted asthe raw material for the invention and gallium is extracted from the flyash via direct acid-leaching process, which saves the step ofcalcination and activation with presence of sodium carbonate at a veryhigh temperature and thus simplifies the procedures and reduces theproduction cost. The acid leaching of the fly ash occurs inacid-resistant reactor at a moderate temperature (in the range of100-200° C.), and thus the leaching efficiency of gallium is high, being80% or more. The effective adsorption efficiency of gallium inhydrochloric leachate is 96% or more when lower-cost macro-porouscationic resin is used for adsorbing gallium. During enriching galliumin the hydrochloric leachate by the macro-porous cationic resin, iron inthe hydrochloric leachate is also effectively removed, so as to obtain arefined aluminum chloride solution with low iron content which can beused for preparing aluminum chloride crystal and metallurgical-gradealumina with low iron content.

In addition, the experimental study has indicated that, since themagnetic separation apparatus according to the present invention isused, the iron removing efficiency is improved by 20% or more, and theiron removing rate is improved from 60% to 80%, which significantlyrelieves the burden of de-ironing from solution in the subsequentprocesses, and thereby reducing the production cost and improving theproduction efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of the method according to the presentinvention;

FIG. 2 is a flow diagram of the multiple-stage carbonation processaccording to the present invention;

FIG. 3 is a schematic diagram of the vertical ring magnetic separatorused in one preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter the method according to the present invention will be furtherdescribed in detail with reference to the drawings, however, it shouldbe understood that the present invention is not limited thereto in anyway.

The structure of vertical ring magnetic separator used for the followingExamples is shown in FIG. 3. The vertical ring magnetic separatorcomprises a rotating ring 101, an inductive medium 102, an upper ironyoke 103, a lower iron yoke 104, a magnetic exciting coil 105, a feedingopening 106 and a tailing bucket 107, and also comprises a pulsatingmechanism 108 and a water washing device 109.

The rotating ring 101 is a circular ring shaped carrier in which theinductive medium 102 is carried. When the rotating ring 101 is rotated,the inductive medium 102 and the matters adsorbed thereon move together,so as to separate the adsorbed matters. The rotating ring 101 may bemade of any suitable material, such as carbon steel etc.

An electric motor or other driving device can provide power to therotating ring 101 such that the rotating ring 101 can rotate in a setspeed.

When parameters, such as iron content or treating amount of the materialto be treated is lower than a predetermined value, a relatively lowrotating speed, such as 3 rpm, may be used, in order to make theferromagnetic impurities having sufficient time to be adsorbed onto theinductive medium meshes under the act of magnetic field, and beingseparated.

The inductive medium 102 is arranged in the rotating ring 101. Themagnetic field generated by the magnetic exciting coil 105 makes theupper iron yoke 103 and the lower iron yoke 104 to be a pair of magneticpoles generating magnetic field along the vertical direction. The upperiron yoke 103 and the lower iron yoke 104 are arranged at the inner andouter sides of the lower portion of the rotating ring 101 such that therotating ring 101 rotates vertically between the magnetic poles. Whenthe rotating ring 101 rotates, the inductive medium 102 in the rotatingring 101 will pass the pair of magnetic poles made up of the upper ironyoke 103 and the lower iron yoke 104 and be magnetized for removing theiron.

The inductive medium 102 may be layers of steel plate meshes. The steelplate meshes are made of 1Cr17. Each layer of steel plate meshes iswoven by wires, with the mesh grid having a rhomb shape. The edges ofthe wires have prismatic sharp angles. The upper iron yoke 103 iscommunicated with the feeding opening 106 and the lower iron yoke 104 iscommunicated with the tailing bucket 107 which is used for dischargingmaterials. The steel plate meshes have a medium layer spacing of 3 mm.The magnetic exciting coil 105 is formed of flat wire solenoid coilwhich is double glass envelope enamelled aluminum and is solidconductor. The current passing through the magnetic exciting coil 105 iscontinuously adjustable, and thus the strength of the magnetic fieldgenerated by the magnetic exciting coil 105 is also continuouslyadjustable.

The vertical ring magnetic separator further comprises a pulsatingmechanism 108 coupled with the tailing bucket 107 via a rubber plate111. The pulsating mechanism can be achieved by an eccentric linkmechanism, such that the alternating force generated by the pulsatingmechanism 108 pushes the rubber plate 111 to move forth and back, it ispossible for the mineral slurry in the tailing bucket 107 to generatepulsations.

The water washing device 109 is arranged above the rotating ring 101,for flushing the magnetic particles into the concentrate hopper by waterflow. The water washing device 109 may be various suitable flushing orspraying device, such as a spraying nozzle, water pipe, etc.

The feeding opening 106 is communicated with a side of the upper ironyoke 103, such that the fly ash can pass through the rotating ring. Thefeeding opening 106 may be a feeding hopper or a feeding pipe. Thefeeding opening 106 is configured for feeding the mineral slurry, suchthat the mineral slurry enters the upper iron yoke 103 with a relativelysmall fall for preventing the magnetic particles from penetrating theinductive medium 102 due to gravity, thus improving the effect ofmagnetically separating and impurities removing.

The vertical ring magnetic separator further comprises a cooling device112, which is provided adjacent to the magnetic exciting coil fordecreasing the working temperature of the magnetic exciting coil. Thecooling device is a pressure balance chamber water jacket. The pressurebalance chamber water jacket is made of stainless steel material, andthus is not prone to scale. As pressure balance chambers arerespectively mounted to the inlet and outlet of the water jacket, theyensure that the water flows uniformly through each layer of water jacketand fills throughout the inside of the jacket, thus preventing any localwater from taking a shortcut which otherwise would affect heatdissipation. Each layer of water jacket has a water passage with a largecross-section area, and thus it is possible to completely avoid blockingdue to scaling. Even if there is a block somewhere, the normal flowingof the circulating water in the water jacket will not be affected.Moreover, the water jacket is in close contact with the coil by a largecontacting area, thus most heat generated by the coil can be taken awayby the water flow.

The pressure balance chamber water jacket, as compared with the commonhollow copper tube for heat dissipation, shows high heat dissipationefficiency, small temperature rise of the windings, and low excitingpower. In case of a rated exciting current of 40 A, the magneticseparator with the pressure balance chamber water jacket for heatdissipation can be reduced from 35 kw to 21 kw.

When the magnetic separator apparatus is working, the fed mineral slurryflows along a slot of the upper iron yoke 103 then through the rotatingring 101. As the inductive medium 102 in the rotating ring 101 ismagnetized in the background magnetic field, a magnetic field with veryhigh magnetic induction strength (such as 22,000 Gs) is formed at thesurface of the inductive medium 102. The magnetic particles in themineral slurry, under the effect of the very high magnetic field, areadhered to the surface of the inductive medium 102, and rotated with therotating ring 101 going into the region without magnetic field at top ofthe rotating ring 101. Then, the magnetic particles are flushed into theconcentrate hopper by the water washing device 109 located above the topof the rotating ring. The non-magnetic particles flow along the slots ofthe lower iron yoke 104 into the tailing bucket 107 and then aredischarged via a tailing exit of the tailing bucket 107.

Hereafter the method according to the present invention will be furtherdescribed in detail with reference to the Examples, however, it shouldbe understood that the present invention is not limited thereto in anyway.

In the following Examples, the circulating fluidized-bed fly ashdischarged by a thermal power plant is used as the raw material and itschemical components are shown in Table 1. The gallium content in the flyash is 0.0042 wt %.

TABLE 1 Chemical components of circulating fluidized-bed fly ash (wt %)SiO₂ Al₂O₃ TiO₂ CaO MgO TFe₂O₃ FeO K₂O Na₂O LOS SO₃ Total 34.70 46.281.48 3.61 0.21 1.54 0.22 0.39 0.17 7.17 1.32 95.77

Example 1

The experimental procedures used in the example are as follows.

(1) Crushing the circulating fluidized-bed fly ash to a size of 200mesh, removing iron by wet magnetic separation using the verticalmagnetic separator as illustrated in FIG. 3, such that the ferric oxidecontent in the fly ash was reduced to 0.8 wt %; putting the filteredcake of the fly ash obtained after magnetic separation into anacid-resistant reactor and adding industrial hydrochloride acid having aconcentration of 37 wt % therein to perform acid dissolving reaction,wherein the molar ratio of HCl contained in the hydrochloride acid toalumina contained in the fly ash was 4.5:1, the reaction temperature was200° C., the reaction pressure was 2.1 MPa and the reaction time was 1hour; and then pressure-filtering the discharged reaction product bymeans of plate-and-frame filter press and washing to yield ahydrochloric leachate having pH of 1.7, wherein the leaching efficiencyof gallium from the fly ash was measured to be 84.2%.

(2) Cooling the hydrochloric leachate till its temperature was 65° C. bymeans of heat-exchange, then pumping the hydrochloric leachate throughcorrosion-resistant pump into resin column (single-column and loadedwith D001 Resin (Anhui Wandong Chemical Plant)) to enrich gallium,wherein the flow flux of the hydrochloric leachate was 2 times overresin volume per hour; and when the adsorption reached saturation,eluting the resin column with 4 wt % hydrochloride acid as eluting agentat 25° C. to obtain gallium-rich eluent, wherein the flow flux of thehydrochloride acid was 2 times over resin volume per hour, and the totalamount of the eluting agent used for elution was 2 times over the volumeof the resin; and regenerating the resin with 4 wt % hydrochloride acid,wherein the adsorption efficiency of gallium in the acid leachate wasmeasured to be 96.4%.

(3) Adding 180 g/l sodium hydroxide solution into the eluent, so thatthe mass ratio of alumina to sodium hydroxide in the solution was 1.0,and keeping reacting at 20° C., subjecting the reaction product tofiltration to remove ferric hydroxide precipitate to obtain agallium-containing sodium metaaluminate solution.

(4) Introducing carbon dioxide gas with a flow rate of 80 ml/min into100 ml of the gallium-containing sodium metaaluminate mother solutionobtained from step (3) at 65° C., the pH value at the end of reactionwas 10.5, then filtering the resultant to finish the primarycarbonation; subjecting the filtrate obtained from the primarycarbonation to the secondary carbonation: further introducing carbondioxide gas with a flow rate of 100 ml/min at 60° C., the pH value atthe end of the reaction was 9.8, then filtering the resultant to obtaina gallium-aluminum double salt precipitate. The mass ratio of gallium toalumina in the double salt was 1/330. The gallium content was measuredin accordance with the method of Standard of the People's Republic ofChina GB/T 20127.5-2006 “Steel and Alloy-Determination of Trace ElementContents Part V: Determination of Gallium Content by ExtractionSeparation-Rhodamine B Photometric Method”. The aluminum hydroxidecontent was 100% minus gallium hydroxide content, and thereby thealumina content was calculated.

(5) Adding the aluminum-gallium double salt obtained from step (4) intoa sodium hydroxide solution of 180 g/l and keeping the reaction at 25°C. to obtain a base solution rich of gallium, then adjusting the galliumcontent to 1.5 mol/l and electrolyzing with platinum electrodes as thenegative and positive electrodes, the electrolysis current of 200 mA/l,electrolysis voltage of 4V and electrolytic bath temperature of 40° C.to obtain metal gallium product. The gallium content in the product wasmeasured to be 99.9% according to the method of “YS/T520-2007 Methodsfor Chemical Analysis of Gallium”.

Example 2

The operation conditions were the same as those of Example 1 except step(1). Step (1) was adjusted as follows:

Crushing the circulating fluidized-bed fly ash to a size of 150 mesh,removing iron by wet magnetic separation using the vertical magneticseparator as illustrated in FIG. 3, such that the ferric oxide contentin the fly ash was reduced to 0.8 wt %; putting the filtered cake of thefly ash obtained after magnetic separation into an acid-resistantreactor and adding industrial hydrochloride acid having a concentrationof 28 wt % therein to perform acid dissolving reaction, wherein themolar ratio of HCl contained in the hydrochloride acid to aluminacontained in the fly ash was 5:1, the reaction temperature was 150° C.,the reaction pressure was 1.0 MPa and the reaction time was 2 hours; andthen pressure-filtering the discharged reaction product by means ofplate-and-frame filter press and washing to yield a hydrochloricleachate having pH of 1.5, wherein the leaching efficiency of galliumfrom the fly ash was measured to be 82.8%.

The gallium content in the obtained product was measured to be 99.9%.

Example 3

The operation conditions were the same as those of Example 1 except step(1). Step (1) was adjusted as follows:

Crushing the circulating fluidized-bed fly ash to a size of 200 mesh,removing iron by wet magnetic separation using the vertical magneticseparator as illustrated in FIG. 3, such that the ferric oxide contentin the fly ash was reduced to 0.8 wt %; putting the filtered cake of thefly ash obtained after magnetic separation into an acid-resistantreactor and adding industrial hydrochloride acid having a concentrationof 20 wt % therein to perform acid dissolving reaction, wherein themolar ratio of HCl contained in the hydrochloride acid to aluminacontained in the fly ash was 8:1, the reaction temperature was 100° C.,the reaction pressure was 0.1 MPa and the reaction time was 4 h; andthen pressure-filtering the discharged reaction product by means ofplate-and-frame filter press and washing to yield a hydrochloricleachate having pH of 1.4, wherein the leaching efficiency of galliumfrom the fly ash was measured to be 80.1%.

The gallium content in the obtained product was measured to be 99.9%.

Example 4

The operation conditions were the same as those of Example 1 except step(2). Step (2) was adjusted as follows:

Cooling the hydrochloric leachate till its temperature was 90° C. bymeans of heat-exchange, then pumping the hydrochloric leachate throughcorrosion-resistant pump into the resin columns (two columns in seriesand loaded with JK008 Resin (Anhui Wandong Chemical Plant)) to enrichgallium, wherein the flow flux of the hydrochloric leachate was 4 timesover resin volume per hour; and when the adsorption reached saturation,eluting the resin column with 2 wt % hydrochloride acid as eluting agentat 60° C. to obtain gallium-rich eluent, wherein the flow flux of thehydrochloride acid was 1 time over resin volume per hour, and the totalamount of the eluting agent used for elution was 2 times over the volumeof the resin and 4 wt % hydrochloride acid was used for the regenerationof the resin, wherein the adsorption efficiency of gallium in the acidleachate was measured to be 96.9%.

The gallium content in the obtained product was measured to be 99.9%.

Example 5

The operation conditions were the same as those of Example 1 except step(2). Step (2) was adjusted as follows:

Cooling the hydrochloric leachate till its temperature was 70° C. bymeans of heat-exchange, then pumping the hydrochloric leachate throughcorrosion-resistant pump into the resin columns (two columns in seriesand loaded with 732 Resin (Anhui Sanxing Resin Ltd., Co)) to enrichgallium, wherein the flow flux of the hydrochloric leachate was 1 timeover resin volume per hour; and when the adsorption reached saturation,eluting the resin column with water as eluting agent at 60° C. to obtaingallium-rich eluent, wherein the flow flux of the water was 1 time overresin volume per hour, and the total amount of the eluting agent usedfor elution was 3 times over the volume of the resin and the adsorptionefficiency of gallium in the acid leachate was measured to be 96.2%.

The gallium content in the obtained product was measured to be 99.9%.

Example 6

The operation conditions were the same as those of Example 1 except step(2). Step (2) was adjusted as follows:

Cooling the hydrochloric leachate till its temperature was 40° C. bymeans of heat-exchange, then pumping the hydrochloric leachate throughcorrosion-resistant pump into the resin column (single-column form andloaded with SPC-1 Resin (Shanghai Resin Plant)) to enrich gallium,wherein the flow flux of the hydrochloric leachate was 1 time over resinvolume per hour; and when the adsorption reached saturation, eluting theresin column with 10 wt % hydrochloride acid as eluting agent at 30° C.to obtain gallium-rich eluent, wherein the flow flux of thehydrochloride acid was 3 times over resin volume per hour, and the totalamount of the eluting agent used for elution was 1 time over the volumeof the resin and the adsorption efficiency of gallium in the acidleachate was measured to be 96.5%.

The gallium content in the obtained product was measured to be 99.9%.

Example 7

The operation conditions were the same as those of Example 1 except step(3). Step (3) was adjusted as follows:

Adding 240 g/l sodium hydroxide solution into the eluent, so that themass ratio of alumina to sodium hydroxide in the solution was 2, andkeeping reacting at 90° C., subjecting the reaction product tofiltration to remove ferric hydroxide precipitate to obtain agallium-containing sodium metaaluminate solution.

The gallium content in the obtained product was measured to be 99.9%.

Example 8

The operation conditions were the same as those of Example 1 except step(4). Step (4) was adjusted as follows:

Introducing carbon dioxide gas with a flow rate of 160 ml/min into 100ml of the gallium-containing sodium metaaluminate mother solutionobtained from step (3) at 90° C., the pH was controlled to 9.8, thenfiltering the resultant to finish the primary carbonation; subjectingthe filtrate obtained from the primary carbonation to the secondarycarbonation: further introducing carbon dioxide gas with a flow rate of150 ml/min at 60° C., the pH was controlled to 9.0, then filtering theresultant to obtain gallium-aluminum double salt precipitate. The doublesalt was dissolved in the sodium metaaluminate mother solution, andunder the same conditions, the above primary and secondary carbonationswere repeated to obtain gallium-aluminum double salt precipitate again.The mass ratio of gallium to alumina in the latest double salt wasmeasured to be 1/290.

The gallium content in the obtained product was measured to be 99.9%.

Example 9

The operation conditions were the same as those of Example 8 except step(4). In step (4), after the twice carbonations as described in Example8, under the same conditions, the carbonation was repeated for the thirdtime to obtain a gallium-aluminum double salt precipitate. The massratio of gallium to alumina in the double salt was measured to be 1/120.

The gallium content in the obtained product was measured to be 99.9%.

Example 10

The operation conditions were the same as those of Example 8 except step(5). Step (5) was adjusted as follows:

Adding the gallium-aluminum double salt obtained from step (4) into asodium hydroxide solution with a concentration of 240 g/l, and keepingreacting at 25° C. to obtain a base solution rich of gallium, thenadjusting the gallium content to 1.1 mol/l and electrolyzing the basesolution to obtain metal gallium product.

The gallium content in the obtained product was measured to be 99.9%.

1. A method for extracting gallium from fly ash, comprising thefollowing steps: a) crushing the fly ash to a size of 100 mesh orsmaller, removing iron by wet magnetic separation, such that the ferricoxides content in the fly ash is reduced to 1.0 wt % or less, thenadding hydrochloride acid into the de-ironed fly ash to perform anacid-leaching reaction, and subjecting the reaction product tosolid-liquid separation to yield a hydrochloric leachate having a pHvalue in the range of 1-3; b) adsorbing gallium in the hydrochloricleachate by passing the same through a column loading with amacro-porous cationic resin; eluting the column with water orhydrochloride acid as an eluting agent when the adsorption reachessaturation to obtain a gallium-containing eluent; c) adding sodiumhydroxide solution into the gallium-containing eluent to react,separating precipitates after reaction by filtration to obtain agallium-containing sodium metaaluminate solution; d) subjecting thegallium-containing sodium metaaluminate solution to carbonation byintroducing carbon dioxide therein, and then separating gallium frommost aluminum to obtain a gallium-aluminum double salt with the massratio of gallium to alumina being more than 1:340; and e) adding thegallium-aluminum double salt into a sodium hydroxide solution,subjecting the reactant to evaporation and concentration to obtain abase solution containing gallium and aluminum with the contents ofgallium and alumina being μmol/l or more respectively, and thenelectrolyzing the base solution to obtain metal gallium, wherein in theacid leaching reaction of step a), the reaction temperature is 100-200°C., the reaction pressure is 0.1-2.5 MPa, and in step b), themacro-porous cationic resin is selected from any one of D001, 732 and742.
 2. The method according to claim 1, wherein, in step a), theconcentration of the hydrochloride acid is 20-37 wt %; the molar rationof HCl contained in the hydrochloride acid to alumina contained in thefly ash is 4:1-9:1.
 3. The method according to claim 2, wherein, in theacid-leaching reaction of step a), reaction time is 0.5-4.0 hours. 4.(canceled)
 5. (canceled)
 6. The method according to claim 3, wherein, instep b), adsorbing gallium in the hydrochloric leachate by passing thehydrochloric leachate through the column from the bottom to top with avolume flux of 1-4 times over resin volume per hour at 20-90° C.
 7. Themethod according to claim 6, wherein, in step b), eluting saidmacro-porous cationic resin with 2-10 wt % hydrochloride acid as aneluting agent, and preferably, the eluting temperature is 20-60° C., theamount of the eluting agent used is 1-3 times over the volume of theresin, and the eluting rate is 1-3 times over resin volume per hour. 8.The method according to claim 1, wherein, in step c), the concentrationof sodium hydroxide solution is 180-240 g/l; preferably, the reactiontemperature is 20-100° C.
 9. The method according to claim 1, wherein,in step d), the carbonation by introducing carbon dioxide into thegallium-containing sodium metaaluminate solution comprises the steps of:performing a primary carbonation: introducing carbon dioxide into thegallium-containing sodium metaaluminate mother solution obtained in stepc), in which the flow rate of carbon dioxide is in the range of 80-160ml/min, the reaction temperature is controlled in the range of 40-90°C., the carbonation time is in the range of 4-10 h, the pH value at theend of the reaction is in the range of 10.6-9.7, then separating theprecipitates from the solution by filtration, so as to separate galliumfrom aluminum for the first time; performing a secondary carbonation:further introducing carbon dioxide into the solution obtain from theprimary carbonation after the separation of the aluminum hydroxideprecipitates, in which the flow rate of carbon dioxide is in the rangeof 100-160 ml/min the reaction temperature is controlled in the range of30-60° C., the carbonation time is in the range of 3-7 h, the pH valueat the end of the reaction is in the range of 9.8-9.0, so as toprecipitate all aluminum and most gallium; subjecting the reactant tofiltration to obtain gallium-aluminum double salt; then crystallizingsodium carbonate in the filtrate obtained from the filtration byevaporization and concentration and separating the crystallized sodiumcarbonate from the solution; and then recycling the filtrate containinga small amount of gallium obtained after the separation of sodiumcarbonate to the beginning of the secondary carbonation for furthercarbonation.
 10. The method according to claim 9, wherein, in step d),when the mass ratio of gallium to alumina in the gallium-aluminum doublesalt obtained after the primary carbonation and secondary carbonation isequal to or less than 1:340, dissolving the double salt in a sodiumhydroxide solution or the sodium metaaluminate mother solution andrepeating the primary carbonation and the secondary carbonation untilthe mass ratio of gallium to alumina in the last gallium-aluminum doublesalt is more than 1:340.
 11. The method according to claim 1, wherein,in step e), the concentration of sodium hydroxide solution is 180-245g/l; preferably, the reaction temperature in step e) is 20-100° C. 12.The method according to claim 1, wherein, in step e), when the basesolution containing aluminum and gallium is electrolyzed, platinumelectrodes are used as the negative and positive electrodes,electrolysis current is in the range of 180-200 mA/l, electrolysisvoltage is in the range of 4V and electrolytic bath temperature is inthe range of 35-45° C.
 13. The method according to any claim 1, wherein,in step a), the apparatus used for de-ironing by wet magnetic separationis a vertical ring magnetic separator which comprises a rotating ring,an inductive medium, an upper iron yoke, a lower iron yoke, a magneticexciting coil, a feeding opening, a tailing bucket and a water washingdevice, wherein the feeding opening is used for feeding the coal ash tobe de-ironed, the tailing bucket is used for discharging thenon-magnetic particles after de-ironing, the upper iron yoke and thelower iron yoke are respectively arranged at the inner and outer sidesof the lower portion of the rotating ring, the water washing device isarranged above the rotating ring, the inductive medium is arranged inthe rotating ring, the magnetic exciting coil is arranged at theperiphery of the upper iron yoke and the lower iron yoke so as to makethe upper iron yoke and the lower iron yoke to be a pair of magneticpoles for generating a magnetic field in the vertical direction, andwherein the inductive medium is layers of steel plate meshes, each steelplate mesh is woven by wires, and the edges of the wires have prismaticsharp angles.
 14. (canceled)
 15. The method according to claim 13,wherein the vertical ring magnetic separator further comprises apressure balance chamber water jacket disposed adjacent to the magneticexciting coil.
 16. The method according to claim 15, wherein the steelplate mesh has a medium layer spacing of 2-5 mm, preferably 3 mm; andthe steel plate mesh is made of 1Cr17.
 17. The method according to claim16, wherein the steel plate mesh has a thickness of 0.8-1.5 mm, a meshgrid size of 3 mm×8 mm-8 mm×15 mm, and a wire width of 1-2 mm,preferably, the steel plate mesh has a thickness of 1 mm, a mesh gridsize of 5 mm×10 mm, and a wire width of 1.6 mm.
 18. The method accordingto claim 17, wherein the vertical ring magnetic separator furthercomprises a pulsating mechanism, which is coupled with the tailingbucket via a rubber plate.
 19. The method according to claim 18, whereinthe inductive medium is provided in the entire circle of the rotatingring.
 20. The method according to claim 19, wherein the magneticexciting coil is a flat wire solenoid coil which is double glassenvelope enamelled aluminum.
 21. The method according to claim 20,wherein the magnetic field strength of the vertical ring magneticseparator is 15,000 Gs or more, preferably 15,000-20,000 Gs, furtherpreferably 15,000-17,500 Gs.
 22. A method for extracting gallium fromfly ash, comprising the following steps: a) crushing the fly ash to asize of 100 mesh or smaller, removing iron by wet magnetic separation,such that the ferric oxides content in the fly ash is reduced to 1.0 wt% or less, then adding hydrochloride acid into the de-ironed fly ash toperform an acid-leaching reaction, and subjecting the reaction productto solid-liquid separation to yield a hydrochloric leachate having a pHvalue in the range of 1-3; b) cooling the hydrochloric leachate till itstemperature is 90° C., then pumping the hydrochloric leachate into acolumn loaded with JK008 Resin to enrich gallium, wherein the flow fluxof the hydrochloric leachate is 4 times over resin volume per hour; andwhen the adsorption reached saturation, eluting the column with 2 wt %hydrochloride acid as an eluting agent at 60° C. to obtain agallium-rich eluent, wherein the flow flux of the hydrochloride acid is1 time over resin volume per hour, and the total amount of the elutingagent used for elution is 2 times over the volume of the resin; c)adding sodium hydroxide solution into the gallium-containing eluent toreact, separating precipitates after reaction by filtration to obtain agallium-containing sodium metaaluminate solution; d) subjecting thegallium-containing sodium metaaluminate solution to carbonation byintroducing carbon dioxide therein, and then separating gallium frommost aluminum to obtain a gallium-aluminum double salt with the massratio of gallium to alumina being more than 1:340; and e) adding thegallium-aluminum double salt into a sodium hydroxide solution,subjecting the reactant to evaporation and concentration to obtain abase solution containing gallium and aluminum with the contents ofgallium and alumina being 1 mol/l or more respectively, and thenelectrolyzing the base solution to obtain metal gallium, wherein in theacid-leaching reaction of step a), the reaction temperature is 100-200°C., the reaction pressure is 0.1-2.5 MPa.
 23. A method for extractinggallium from fly ash, comprising the following steps: a) crushing thefly ash to a size of 100 mesh or smaller, removing iron by wet magneticseparation, such that the ferric oxides content in the fly ash isreduced to 1.0 wt % or less, then adding hydrochloride acid into thede-ironed fly ash to perform an acid-leaching reaction, and subjectingthe reaction product to solid-liquid separation to yield a hydrochloricleachate having a pH value in the range of 1-3; b) cooling thehydrochloric leachate till its temperature is 40° C., then pumping thehydrochloric leachate into a column loaded with SPC-1 Resin to enrichgallium, wherein the flow flux of the hydrochloric leachate is 1 timeover resin volume per hour; and when the adsorption reached saturation,eluting the column with 10 wt % hydrochloride acid as an eluting agentat 30° C. to obtain a gallium-rich eluent, wherein the flow flux of thehydrochloride acid is 3 times over resin volume per hour, and the totalamount of the eluting agent used for elution is 1 time over the volumeof the resin; c) adding sodium hydroxide solution into thegallium-containing eluent to react, separating precipitates afterreaction by filtration to obtain a gallium-containing sodiummetaaluminate solution; d) subjecting the gallium-containing sodiummetaaluminate solution to carbonation by introducing carbon dioxidetherein, and then separating gallium from most aluminum to obtain agallium-aluminum double salt with the mass ratio of gallium to aluminabeing more than 1:340; and e) adding the gallium-aluminum double saltinto a sodium hydroxide solution, subjecting the reactant to evaporationand concentration to obtain a base solution containing gallium andaluminum with the contents of gallium and alumina being μmol/l or morerespectively, and then electrolyzing the base solution to obtain metalgallium, wherein in the acid-leaching reaction of step a), the reactiontemperature is 100-200° C., the reaction pressure is 0.1-2.5 MPa.